Corrosion preventive galvanic anode zinc alloy



United States Patent CORROSION PREVENTIVE GALVANIC ANODE .znvc AL'LOY Takeshi Sakano, Urawa-shi, SaitamaJ-keu, Akira Masuda Omiya-shi, Saitama-ken, Kazuo Toda, Urawa-slu, Saitama-ken, and Yoriyasu Atobe, 0miyashi, Sartama-keu," Japan, assiguors to Mitsubishi Kenzoku,

The present invention relates to the prevention of corrosion of ferrous metals, and more particularly to an improved zinc alloy to be used as the anode material for the purpose of preventing corrosion of ship bodies, iron structures or the like.

Heretofore, the galvanic anode serving to protect ship bodies, iron structures or the like from corrosion, generally were met-alssuch as -high purity zinc, magnesium, magnesium alloys, aluminum alloys, etc., among which, high purity zinc has been widely employed. 4 The employment of -zinc as aga'lvanic anode .has been attempted for many years. However, it has not always been generally used due to thefact that a sufficient corrosion preventive effect .could never-"be attained by the zinc formerly used, because the current-supplying property of zinc is inferior to those of some other metals such as, for example, magnesium or magnesium alloys.

As is well known, "the normal. electrode potential of pure zinc is l.0 1 Volt (saturated calomel electrode) which is subtant'ially lowvas compared with that of iron, namely -0.69 volt. Accordingly, from the theoretical point of viewfa suflicient corrosion preventive current can .be supplied by use -of zinc as an agent for preventing corrosion to iron'structures. However, when conventional zinc is used asa galvanic anode in sea Water, anode potential rapidly rises with time to such a degree that it ;fails to function as a protective anode and the reason thereof may be ascribed to the fact that a coating of the .corrosion product. is massively deposited on the surface of said anode, therebymaking said surface highly resistive to the flow of electric current. And it is well known that the greater the amount of impurity contained in the zinc, the greater the above stated tendency becomesz Thestudymfthe deterioration of the-zinc as described above resulted in the-suggestion of a zinc electrode of high purity." -Recentlythe technique of'refining zinc has progressed-soreinarkably that a zinc product canreadily be manufactured the purity of which is as high as 99,995% to 99.999%. Employment of zinc having such Qhigh purity [can noticeably reduce said deterioration "causedlby the rising of anode potential by maintaining the anode at asufi iciently low potential for a long period .of 'time; :thereby attaining a substantial efiectiveness in preverjiti' igpdrrosim; Forfexamplei employment 'of a zinc anode in sea water for 10001101518 resulted in a po- Patented May 2, 1961 be regarded as the material for galvanic anodes which is very practical for preventing corrosion to ship bodies, iron structures or the like, because of the cost of said zinc is much' lower than that of magnesium or' alloys thereof; v i

As the results of experiments which were carried out for a long period of time in sea water in connection with galvanic anodes made of zinc, the purity of which is from 99.998% to 99.999%, we have'found that the high purity zinc which has been considered to be superior material for the galvanic anode can not maintain a suflicient corrosion preventive effect over long periods of time.

That is to say, we have observed that even when high purity zinc is used, a corrosion product is depositedon the surface of the metal and the amount of said product increases gradually while it is used, thus causing a gradual rise of anode potential of the zinc, thereby decreasing the effective potential difference which is necessary for generating a protective current and reducing the effect of preventing the corrosion. Furthermore, we have observed by practical tests with ship bodies that the corrosion product covering the anode adheres to the surface so as to form a solid coating and does not readily fall 01f by contact with sea water during voyage, thus causing insufficient dissolution of the zinc electrode. Therefore, it was found that, evenin the case of the high purity which has far better properties as fc'om'pared with such a low grade zinc as conventional distilled zinc, a suflicient -corrosion preventive effect can not always be expected.

" Although many attempts were made to overcome the foregoing difficulties and other disadvantages, none, as

far as we are aware, was entirely successful when car-' ried into practice commercially on an industrial scale.

It has now been discovered that an'alloy can be made having a superior corrosion preventive effect overa long period of time. I

It is an object of the present invention to provide an improved zinc alloy for use as a galvanic anode, which is superior to the zinc of high purity from the standpoint of reduction of the corrosion' product which is adherently the center of group III of the periodic table '(in accordance with the Periodic-Chart of Elements vfound on pages 342 and 343.Handbook.of Cheniistry and Physics, 33rd edition, 1951 and 1952, published by Chemical'Rubber Publishing 00., Cleveland, Ohio) (or other editions of the same work). Thus, theuseful metals of the group for our purposes can be described as indium and the metals tential" rise of more than 0.8 volt in the case"of the a of which was 99.99%, anode poten of between 41.00 and 0.95

se of said pure zinc as the;galv anic a'node Pun'ty was 99.6%, butin'the case of 1" d fi r w 1 e gniz s p abo e desc bed. 'z'zinqmaterials'q' In ac: i -cordance with the apprec ation of the'pr'o'perty of zinc pf. V i 'h g lr i yz i i {{i e., elernentslnumber 48,49; ,metals having a low' melting below and. on' both sides of indium (referring to the above mentioned handbook, as follows :j

These metals having similar properties have a similar elfect in improving the property of high purity zinc, so that when two or more of said metals are added together, i.e., vused together, equal ,or-multiplied eflect will be realized.

. The continuous maintenanceofsuch an anode potential, i.e., 1 .00- 1 .02 volt, (saturated calomel electrode) as that obtained at the beginning of current supply and the possibility of supplying a sufficient anti-corrosion current which is necessary forprotecting the iron structures from corrosion are desirable properties for the galvanic anode to be used in sea water. For obtaining the abovestated properties, it is necessary not only to make the amount of corrosion product beingdeposited on the'sur- ,face of the zinc anode small, but also to avoid the rigid adherence of said product to the surface of said anode.

The Zinc alloy of this invention is an excellent material for the current-supplying anode, because the corrosion product on the surface of the zinc alloy which is the subject of the present invention will only amount to between 1/3 and /2 of that product in the case of the high purity .zinc, even after a long period of time under current supplying conditions. Furthermore, the corrosion product on said zinc alloy is in a soft state similar to that of jelly and can easily be scraped away, the anode potential is always maintained at a value of the order of -1.02 volt, i.e., the anode potential at the initial period of electric current supply, thus anode metal.

causing uniform solubility of the In carrying theinvention into practice, it is preferred to use a zinc of-the highest purity, above about 99.99% pure and one of the metals of the group of metals near the center of group III A of the periodic table, i.e., elements numbers 48, 49, 50 and 81, or a plurality thereof, in a range of from-about 0.01% to about 3%, based on the weight of the zinc.

For the purpose of giving those skilled in the art a better understanding of the invention and a better appreciation of the advantages of the invention, the following illustrative examples are g ven:

' EXAMPLE I Saturated calomel electrode Corrosion Anode poten- Anode potenproduct,

tial at the tial after Percent beginning of 2,000 hours, current supply, volts volts High purity zinc (99. 999%)- 1.02-1. 03 0. 97--0. 89 11-20 High purity zinc plus cadmium:

Cadmium, percent- EXAMPLE II Saturated calornel electrode Corrosion Anode poten- Anode potenproduct,

tial at the tial after Percent beginning'of 2,000 hours,

- current supply, volts volts High purity zinc plus indium: V

Indium, percent' i 1'.O2-1. 03 1.01 1 02--1.03 1.03 -1.02-,-1.03 -1.02 k 4 l 02-1.'03 1.02 7

jabpm 0.013%

EXAMPLE III i Saturated calomel electrode Corrosion :A nodelpoten- Anode potenproduct,

tial at the tlal after Percent 1 beginning of- -2,000hours;. currentsupply; volts volts 1 High purity zinc plus thal- I liurn:

Thallium, percent EXAMPLE IV.

Saturated calomel electrode v Corrosion Anode poten- Anode'potenproduct,

tial at the i tial after Percent beginning of 2,000 hours, currentsupply. volts volts J High purity zinc plus tin:

Tin, IJQICBD-trt EXA PIIE" Saturated ealornel electrod Corrosion Anode poten- Anodepotenpruduct,

tial at the tial after Percent 1 beginning of 2,000 hours currentsupply, volts.

volts High purity zinc plus cadmium plus indium plus thallium plus tin:

, garniuru,0( )7. 5%. a

llILUl, Thamum, L6 }-1. 02--1. 03 --1. 01 7 Tin, 0.2%--..

1 Amount of corrosion product adhered to anodeX J Amount of dissolved zinc 1 It is to be noted that the presentinvention, is not; to be confused with the addition of other metals to Zinc,..e.g-., lead or bismuth. Although when a suitableamount of lead orbismuth is added independently, the property of the pure zinc witllbe irnprovedsomewhat, thegimprovement so obtained is liar belowthait ofthe elloy of this invention- Although the present invention has been described in conjunction with preferred embodiments, it, is to be understood that modifications and variationsmay be resorted to-Without. departing from the'spiritand scopeof the in vention, as r'thos'eskilled in the d1 underlclaimsg V We claim g 1 [Co"rrosionpreventive galva c 'ano d zinc, is abovelab'ou 9 9 at least one element selected the group elements nuinbe" 8 9, 8 O t 2,982,706 5 6 about 99.99% and at least one element selected from the selected from the group is mixtures of indium, thallium, group consisting of indium, thallium, cadmium, tin, and Oadmium'alld mixtures thereof, 1n an amount of between about 0.01% References Cited in the file of this Patent and about 3 based on the weight of the zinc.

3. A method as claimedin claim 2, wherein the element 5 UNITED STATES PATENTS selected from the group is indium. Re. 4,048 Lighthall June 28, 1870 4. A method as claimed in claim 2, wherein the element 2,444,174 Tarr et a1 June 29, 1948 selected fIOIn the group iS thallium. FOREIGN PATENTS 5. A method as claimed in claim 2, wherein the element Selected from the group is Cadmimm 10 15,040 7 Great Bntam Dec. 8, 1885 6. A method as claimed in claim 2, wherein the element OTHER REFERENCES Seleoted from the group is tl'11- Metals Handbook, Am. Soc. for Metals, 19-48, pages 7. A method as claimed in claim 2, wherein the element 107 7 and 1083. 

2. A METHOD OF PREVENTING CORROSION TO IRON STRUCTURES WHICH COMPRISES THE STEPS OF USING AS A GALVANIC ANODEAN ALLOY CONSISTING OF ZINC THE PURITY OF WHICH IS ABOVE ABOUT 99.99% AND AT LEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING OF INDIUM, THALLIUM, CADMIUM, TIN, AND MIXTURES THEREOF, IN AN AMOUNT OF BETWEEN 0.01% AND ABOUT 3% BASED ON THE WEIGHT OF THE ZINC. 